The invention relates to pharmaceutically active macrolides, synthesis thereof and intermediates thereto. Halichondrin B is a potent anticancer agent originally isolated from the marine sponge Halichondria okadai, and subsequently found in Axinella sp., Phakellia carteri, and Lissondendryx sp. A total synthesis of Halichondrin B was published in 1992 (Aicher, T. D. et al., J. Am. Chem. Soc. 114:3162-3164). Halichondrin B has demonstrated in vitro inhibition of tubulin polymerization, microtubule assembly, betaS-tubulin crosslinking, GTP and vinblastine binding to tubulin, and tubulin-dependent GTP hydrolysis and has shown in vitro and in vivo anti-cancer properties. Accordingly, there is a need to develop synthetic methods for preparing analogs of Halichondrin B useful as anti-cancer agents.